1. Field of the Invention:
The present invention relates to an overlock sewing machine for carrying out an overlocking operation. 2. Description of the Prior Art:
A prior art overlock sewing machine includes a chain looper, an upper looper, a lower looper, and two needles positioned aslant in the cloth feeding direction and respectively cooperative with the chain, the upper and the lower loopers, to effect an overlocking operation by two needles with five threads (Stitch #516 (=#401+#504) according to JIS L0120 Classification of Stitching Type and Indication Symbol, hereinafter simply referring to Stitch #). In the prior art overlock sewing machine, it is known that the upper looper is exchanged for an overlock looper to effect an overlocking operation by two needles with four threads (#515 (=#401 +#503)).
It is also known that double-loop stitching operation (Stitch #401) can be effected by use of only the chain looper.
In the overlock sewing machine having the upper looper, the lower looper, and two needles positioned in parallel and crossing the cloth feeding direction, it is a so known by exchanging the upper looper for the spreader to effect an overlocking operation by two needles with three threads (Stitch #509).
That is, stitches formed by the overlock sewing machine having two needles positioned aslant in the cloth feeding direction with five threads and a chain looper are Stitch #516, Stitch #504, Stitch #505 and Stitch #401, and stitches formed by the same sewing machine in which the upper looper is exchanged for the overlock looper are Stitch #515, Stitch #512 and Stitch #503.
Stitches obtained by the overlock sewing machine having two needles crossing the cloth feeding direction, the upper and the lower loopers, and four threads are Stitch #514, Stitch #504 and Stitch #505, and stitches formed by the same sewing machine in which the upper looper is exchanged for the spreader are Stitch #509,Stitch #502 and Stitch #503.
A looper switching unit is recently invented in which a looper switching operation is carried out on an upper looper fixing table or a lower looper fixing table for serving as the spreader as disclosed, for example, in Japanese Patent Publication No. 56-13478. Use of the looper switching unit makes it possible to provide the overlock sewing machine capable of sewing two types of overlocking operations.
However, the prior art overlock sewing machine can not effect both overlocking operations for forming Stitch #514 requiring the arrangement of two needles crossing the cloth feeding direction and for forming the Stitch #516 requiring the arrangement of two needles aslant in the cloth feeding direction. Hence, there occurred such a problem that a plurality of overlock sewing machines are needed for effecting the overlocking operations set forth above.